I am wondering for someone to discuss the phenomena that newborn babies (humans) are able to submerge underwater up until a few months where that ability is lost again? What is the explanation for this?

Another action of the diving response can be observed in infants when they are under water. The windpipe by the vocal chords sponta­neously closes to prevent water from entering the lungs. This reflex is initiated as soon as there is contact with water. However, it disappears when the child reaches the age of roughly six months.

When the face of an infant is exposed to cold water, the heart slows down and blood is shifted away from the peripheral muscles to conserve oxygen for the brain and heart, and they typically hold their breath. The reflex is the same one that protects babies from getting milk in their lungs. The reflex kind of reroutes the entire circulatory system to save blood for the heart and brain, and not pump it through the rest of the body.
http://www.ncbi.nlm.nih.gov/pubmed/12022304